Re: [PATCH v2 00/19] Fixes for sched/numa_balancing

[Peter Zijlstra]

On Mon, Jul 23, 2018 at 09:29:54AM -0700, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2018-07-23 17:21:47]:
> 
> > On Mon, Jul 23, 2018 at 08:09:55AM -0700, Srikar Dronamraju wrote:
> > 
> > > > >   sched/numa: Stop multiple tasks from moving to the cpu at the same time
> > > 
> > > This patch has go-ahead from Mel and Rik and no outstanding comments.
> > 
> > I left it out because it's part of the big xchg() mess.
> > 
> > In particular:
> > 
> > +       if (xchg(&rq->numa_migrate_on, 1))
> > +               return;
> > +
> > +       if (env->best_cpu != -1) {
> > +               rq = cpu_rq(env->best_cpu);
> > +               WRITE_ONCE(rq->numa_migrate_on, 0);
> > +       }
> > 
> > I'm again confused by clearing numa_migrate_on at this point..
> 
> First task choose a cpu to swap/migrate, sets the cpu to best_cpu and
> also numa_migrate_on.  Next it finds a better cpu to swap/move. Now if
> the task is able to move to the better cpu, then it should clear
> numa_migrate_on on the previous best_cpu.
> 
> If we dont reset numa_migrate_on on finding a better cpu, the
> numa_migrate_on stays set for the previous cpu, causing previous cpu to
> never be a target of numa balance.

Don't tell me, write better patches.